Fiber optic communications networks are enhanced and made cheaper by increased integration of components, sub-assemblies, modules and systems. At the sub-assembly level, this may entail incorporating several optoelectronic and optical components such that these components share a common optical path. For example, IEEE standard 802.3ae, published on 13 Jun. 2002, relates to 10 gigabit/second optical Ethernet and calls for four wavelengths (˜1275, 1300, 1325 and 1350 nm) of light to be simultaneously transmitted on a single optical fiber. Fiber optic communications networks also desirably have smaller and cheaper components, sub-assemblies and modules. Continuing with the 10 gigabit/second optical Ethernet example, optical transceiver modules, which convert electrical signals to optical signals on a transmit channel and optical signals to electrical signals on a receive channel, compliant with the IEEE 802.3ae standard are further defined by the manufacturers of these transceivers according to multi-source agreements (MSAs). Several of these MSAs (known by various codes: X2, XPAK, XFP) require small optical transmit/receive sub-assemblies, on the order of 1000 mm3 or less for both functions. Moreover, the historical cost trends for Ethernet and the expected future mass deployment (millions of ports) require that the cost of such sub-assemblies be low. Ideally, the manufacture of such sub-assemblies would be automated.
Continuing with the above example, an optical sub-assembly that performs a multiple-wavelength multiplexing and transmit function may include several laser diodes, a lens (at the output of each laser diode) that focuses or collimates the light of each wavelength, a multiplexer that combines the different wavelengths into a combined optical signal, a lens that focuses the light signal output from the multiplexer and an output optical fiber for distance-transmission of the combined optical signal from the optical sub-assembly. In order for the sub-assembly to function properly, the outputs of the laser diodes must be properly aligned with the lenses, which, in turn, must be properly aligned with the inputs to the multiplexer, and the output of the multiplexer must be aligned with the output lens, which, in turn, must be aligned with the output optical fiber. In the case when the optical fiber is single-mode fiber, as is the case in part of the IEEE 802.3ae standard, the tolerances for this alignment approach ±1 μm, since a typical core diameter of the fiber is 9 μm. Those familiar with the art will recognize that the need to align multiple optoelectronic components (laser diodes, in this example) significantly increases the difficulty of manufacture.
It is known in prior art to align one optoelectronic device to one single-mode fiber, possibly including intermediary optical components such as a lens. The most reliable method for such alignment is “active alignment”, practiced for many years, in which the optoelectronic device is energized and the various components are moved relative to each other in order to obtain an acceptable throughput of optical signal, then the components are secured in place. Wang et al teach in U.S. Pat. No. 6,698,940 an automate-able version of this method, but the method is limited by being applicable to one style of sub-assembly package and is not readily extendable to align multiple optoelectronic components. Another alignment method is “passive alignment”, in which all the components are located by stops, indentations in an optical bench (or substrate), or placed with reference to precision fiducial marks, or other means. By way of example, from among many, Verdiell teaches in U.S. Pat. No. 6,376,268 the use of various steps and raised structures to assist in placement of components, and Chang et al teach in U.S. Pat. No. 6,485,198 the use of balls mating with indentations in components and substrates to assist in placement of components. Such methods would be readily extendable to align multiple optoelectronic components, but, along with much prior art involving passive alignment, these methods are of limited applicability because of the mechanical imprecision of the optoelectronic device die themselves. While the semiconductor layers in optoelectronic devices are controlled in thickness to ˜0.001 μm (1 nm) and the lateral semiconductor and metallization features are lithographically defined with a precision of ˜0.1 μm (100 nm), the thickness of the die and the lateral cutting of the die out of a wafer have imprecision of ±10 μm or considerably more. While it might be possible to improve these tolerances to permit passive alignment, the installed base of optoelectronic production equipment, which gives rise to the ±10 μm tolerances, is so large that such a development is thought to be impractical in the next few years. In addition, a problem often encountered is that various optical components have different heights; for example, a typical optical fiber has a diameter of 125 μm with an optical axis at 62.5 μm height, while a laser diode might be 300 μm tall with an optical emission point essentially at 300 μm height. Co-locating these components on a flat optical bench would result in a mis-match of their optical path heights, the typical solution to which is providing “sub-benches” to raise smaller optical components up to a common-height optical plane. A difficulty arises, however, in that the height of the sub-benches themselves can only be controlled to ˜±10 μm using existing high-precision manufacturing techniques.
A hybrid approach, combining passive and active alignment, is widely used in prior art. In this approach, as many components are passively aligned as practical, particularly including the aforementioned intermediary components between the optoelectronic component and the fiber, then a final active alignment step(s) is performed. By way of example, Von Freyhold et al teach in U.S. Pat. No. 6,616,345 one or more assembly holders, which can be moved over one or more bases along various axes, to bring groups of components into alignment, the components within any one group being passively aligned. Bergmann et al teach in U.S. Pat. No. 6,430,337 an adjustable beam steering device in an otherwise passively aligned optical path. Musk teaches in U.S. Pat. No. 6,445,858 a flexural member upon which a component is mounted such that the component can be brought into alignment with an optical path. Caracci et al teach in U.S. Pat. No. 6,445,838 polymer grippers, which allow a component to be moved to change the cavity length of a Fabry-Perot resonator, while keeping the component passively aligned in both axes transverse to the cavity length. None of these hybrid alignment approaches contemplate aligning several optoelectronic components such that these components share a common optical path.
Prior art does exist for limited cases of a few optoelectronic components sharing a common optical path. The most common types are loosely known as bi-directional optical sub-assemblies, in which a laser diode transmits optical signals in one direction in a fiber and a photo diode receives optical signals traveling in the opposite direction in the same fiber. Usually an optical filter arrangement separates the optic signals by wavelength. Ojima et al in U.S. Pat. No. 6,334,716 teach such a bi-directional sub-assembly. Tsumori in U.S. Pat. No. 6,509,989 and Althaus in U.S. Pat. No. 6,493,121 teach alternate arrangements having three optoelectronic components, adding a second photo diode for a second reception channel. Althaus in U.S. Pat. No. 6,493,121 further teaches arrangements with four, five or more optoelectronic components, generally in transmitter/receiver pairs. It is believed that alignment of these sub-assemblies, even in the simpler cases, involves individual active alignment of each optoelectronic component. According to existing art, this alignment can only readily be done if each optoelectronic component is individually pre-packaged in hermetically-sealed, thermally conductive “TO cans”, with each whole can being moved to accomplish the alignment. Since a TO can has a volume of ˜150 mm3, a collection of these plus the common optical housing will quickly exceed the aforementioned space constraints of transceiver MSAs. In addition, upon incorporating four or more optoelectronic components, the alignment procedure becomes correspondingly more tedious and optical losses and cross-talk between channels become increasingly troublesome.